First cut at nowarn refinement members

Author: som-snytt

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -150,6 +150,9 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
   override def prepareForValDef(tree: ValDef)(using Context): Context =
     if !tree.symbol.is(Deferred) && tree.rhs.symbol != defn.Predef_undefined then
       refInfos.register(tree)
+    tree.tpt match
+    case RefinedTypeTree(_, refinements) => relax(tree.rhs, refinements)
+    case _ =>
     ctx
   override def transformValDef(tree: ValDef)(using Context): tree.type =
     traverseAnnotations(tree.symbol)
@@ -170,11 +173,15 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
   override def prepareForDefDef(tree: DefDef)(using Context): Context =
     def trivial = tree.symbol.is(Deferred) || isUnconsuming(tree.rhs)
     def nontrivial = tree.symbol.isConstructor || tree.symbol.isAnonymousFunction
-    if !nontrivial && trivial then refInfos.skip.addOne(tree.symbol)
+    if !nontrivial && trivial then
+      refInfos.skip.addOne(tree.symbol)
     if tree.symbol.is(Inline) then
       refInfos.inliners += 1
     else if !tree.symbol.is(Deferred) && tree.rhs.symbol != defn.Predef_undefined then
       refInfos.register(tree)
+    tree.tpt match
+    case RefinedTypeTree(_, refinements) => relax(tree.rhs, refinements)
+    case _ =>
     ctx
   override def transformDefDef(tree: DefDef)(using Context): tree.type =
     traverseAnnotations(tree.symbol)
@@ -456,7 +463,7 @@ object CheckUnused:
         if !tree.name.isInstanceOf[DerivedName] then
           pats.addOne((tree.symbol, tree.namePos))
       case tree: NamedDefTree =>
-        if (tree.symbol ne NoSymbol) && !tree.name.isWildcard then
+        if (tree.symbol ne NoSymbol) && !tree.name.isWildcard && !tree.hasAttachment(NoWarn) then
           defs.addOne((tree.symbol, tree.namePos))
       case _ =>
         if tree.symbol ne NoSymbol then
@@ -851,6 +858,17 @@ object CheckUnused:
         args.foreach(traverse)
       case tree => traverseChildren(tree)
 
+  // NoWarn members in tree that correspond to refinements; currently uses only names.
+  def relax(tree: Tree, refinements: List[Tree])(using Context): Unit =
+    val names = refinements.collect { case named: NamedDefTree => named.name }.toSet
+    val relaxer = new TreeTraverser:
+      def traverse(tree: Tree)(using Context) =
+        tree match
+        case tree: NamedDefTree if names(tree.name) => tree.withAttachment(NoWarn, ())
+        case _ =>
+        traverseChildren(tree)
+    relaxer.traverse(tree)
+
   extension (nm: Name)
     inline def exists(p: Name => Boolean): Boolean = nm.ne(nme.NO_NAME) && p(nm)
     inline def isWildcard: Boolean = nm == nme.WILDCARD || nm.is(WildcardParamName)

tests/semanticdb/metac.expect

@@ -3494,7 +3494,7 @@ Text => empty
 Language => Scala
 Symbols => 12 entries
 Occurrences => 33 entries
-Diagnostics => 2 entries
+Diagnostics => 1 entries
 Synthetics => 4 entries
 
 Symbols:
@@ -3550,7 +3550,6 @@ Diagnostics:
 [14:20..14:23): [warning] Alphanumeric method foo is not declared infix; it should not be used as infix operator.
 Instead, use method syntax .foo(...) or backticked identifier `foo`.
 The latter can be rewritten automatically under -rewrite -source 3.4-migration.
-[19:8..19:17): [warning] unused private member
 
 Synthetics:
 [12:2..12:6):user => reflectiveSelectable(*)

tests/warn/i22681.scala

@@ -10,4 +10,8 @@ class C:
     def run() = ()
     def g = v // warn effectively private member is unused
     def t = v // nowarn
-    def u = v // warn despite structural type (TODO work around the limitation, at least for this example)
+    def u = v // nowarn because leaked by refinement
+  val v: Runnable { def u: Int } = new Runnable:
+    private def v = 42 // avoid g judged too trivial to warn
+    def run() = ()
+    def u = v // nowarn because leaked by refinement